A common form of air cleaner is one in which the polluted air is passed through a filter of porous material, so that pollutant particles are collected on one surface of the filter and clean air emerges from the other surface of the filter.
After a period of operation the quantity of pollutant collected on the filter becomes so great as to cut down the ability of the filter to treat the required volume of air, and the filter must be either replaced or renewed, that is, freed of the collected particles. The latter is preferable, as it can be done without shutting down the air cleaning system and the equipment served thereby.
A common way of renewing a filter is by projecting clean air through the filter in a reverse direction, the dislodged particles dropping from the filter to be scavenged or removed by well-known methods. The entire filter may be subjected to a pulse of renewing air at the same time, briefly reversing the normal air flow, or a jet of limited dimensions may be directed at successive limited portions of the filter in a repeated cycle, so that while each minor portion of the filter is being renewed, the remaining portions continue in the normal function, and normal flow is never completely interrupted.
It has been found that particulate matter released from a filter has a tendency to reattach to the filter when the renewing air flow ceases and normal flow resumes, and this reduces the efficiency of the filter.